Back in 1998, the US Congress extended the period of time a copyright can be held. This means that for the past 20 years virtually no media has entered the public domain in the US. Jan 1, 2019 will mark the end of the first 20 year extension.
This also means that on Jan 1 of every new year in the future, even more works will enter the public domain, unless Congress changes things again.
A great thing about Public Domain content is not only is it free, but if you're selling things like templates of any kind or videos to others, you can do so legally with public domain content.
For more details, check out this article: